NO bath water running!!!

Help! We have a single knob tub and shower faucet that is able to rotate 360 degrees without any tension and there is no water coming out of the faucet spout. It worked the way it always did this morning when I gave my girls their bath (which was you never knew which way you had to move the lever to find the hot water). Now it doesn't matter which way you turn the knob there is only a steady stream of water leaking from the spout about the size of a strand of yarn.